A Royal Navy crewman has been shot dead and another is in a life-threatening condition after a shooting on board nuclear submarine HMS Astute.
A third Royal Navy serviceman was arrested after police were called to Southampton docks at 1212 BST.
A crew member shot two of his crew mates before being overpowered, it is believed.
The BBC understands that the leader of Southampton City Council Royston Smith helped to disarm the gunman.
Eleven children were on the quayside at the time of the shooting and were due to board the submarine.
Police are interviewing 30 potential witnesses to what happened.
A spokesman for Southampton City Council confirmed its mayor Carol Cunio, chief executive Alistair Neill and Mr Smith were all on board at the time of the shooting in the vessel's control room.
The three, who were not injured, were not available for comment.
A police spokeswoman said the incident was not linked to terrorism.
She said there was no public safety risk and the area was sealed off.
BBC News understands the arrested man was handed over to Hampshire police by Ministry of Defence police.
An MoD spokesman said: Two Royal Navy personnel have been involved in a firearms incident at Southampton docks where HMS Astute is alongside.
Sadly, one has now died as a result of his injuries. The Royal Navy is now attempting to inform their families as a matter of urgency.
A third Royal Navy serviceman has been arrested by Hampshire Constabulary and is now in custody.
Defence Secretary Liam Fox said: I am greatly saddened to hear of this incident and of the death of a Royal Navy service person in this tragic incident.
It is right and proper that a full police investigation is carried out and allowed to take its course.
My thoughts and sympathies are with those who have been affected and their families.
Shooting another Royal Navy Serviceman, whom he probably served with. And they probably trusted him.
